distribution of a compound in a two-phase system of water and octanol:

where Po and Pw are the fractions of the total soluble compound in the two phases. The Pow relates back to the above physical and chemical factors and have proven to be a strong predictor for toxicity, persistence, bioaccumulation, biomagnification, and other ecotoxicological parameters. Figure 7 shows a number of examples from the literature on log-log linear relationships between bioconcentration and Pow for various chemicals and aquatic organisms.

Diversity Models

A classical question in ecology is: ''Why are there so many species?'' We know that the number of species and the number of individuals depend on both the development in the past and the habitats in the present. We know that a high diversity in habitats also gives us a high diversity in organisms and this relation is also influenced by climate. The number of habitats within a region grows with the size of the region and so does the number of individuals and creates problems in the interpretation. Comparisons of organism distribution islands within a certain region can provide us the data for empirical models which can be used to study the interactions between diversity, number of individuals, area size, and number of habitats.

A study from 44 of the Aland Islands in the Bothnian Sea showed the abundance and diversity of birds in relation to area of the islands and number of habitats. The

Empirical models have been extremely useful in the study of ecosystems and in the development and management of ecosystems due to their predictive power and the simplicity of the models. But there are also many examples of abuse of empirical models. Empirical models are only valid within the range of the data behind. With an expected climatic change many extrapolations of temperature effects on processes in ecosystems are made by expecting the process rates to increase with the temperature following a simple Arrhenius equation: Rt2 = Rt1 X eA x (t2 _ tl)/'tl x t2, where Rt2 is the process rate at the temperature t2, Rt1 is the rate at the temperature tl, and A is the activation energy in joules per mole. Both by approaching the freezing point and by approaching the temperature where proteins begin to be destroyed, the process rate no longer follows the equation. And empirical models may have strong predictive power, but they may mislead us to wrong concepts. Many storks do not necessarily lead to many newborn babies.

See also: Conceptual Diagrams and Flow Diagrams.

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